The best you can do is create a guid yourself and store it in the keychain for your app. That will survive uninstalls and reboots, but not system wipes. If the user logs in to some cloud service that you can create an association with on your server, e.g. Game Center or Facebook, then that id is generally stable across system wipes and you could do a lookup there. Be careful not to violate those services TOS, however.
